Daphoenodon
If you look very closely at the thicket just south of us on the hillside you can see several fox-like dogs hunting rodents. This Nothocyon (“false dog”) seems to have filled approximately the fox niche during the Early Miocene. Some coyote and wolf-sized dogs are in the area too. Daphoenodon (“blood-reeking tooth”) is about coyote size. Temnocyon (“cutting [tooth] dog”), is a little larger, probably substituted for the wolf in the local fauna, and is characterized by its heavy head and long, strong jaws. If we could get a close look at its teeth, we would see that they are like those of the Cape Hunting Dog living today in South Africa.
Let’s move away from the river a half-kilometer (0.3 mile) or so and see if we can find something different. That thicket ahead might produce a couple of rabbits. Look, there at the edge of the thicket: a Nothocyon has caught something. It’s a Meniscomys (“crescent mouse”), an early relative of the living mountain beaver Aplodontia. Today, a single species of Aplodontia, the last of the line, is found only in the mountains of the West Coast. It’s the most primitive living rodent, not related to the Canadian beaver, and sole survivor of a suborder which was the earliest rodent group to evolve. Meniscomys was one of the most prominent members of the group during the Miocene. It had a round furry body, a round head with protruding incisors a bit like a true beaver’s, and no visible tail.
Watch your step. There is the mound of a pocket gopher. It is neither our familiar western gopher Thomomys (“heap mouse”) or the “eastern” pocket gopher of the Great Plains, Geomys (“earth mouse”), but an ancient relative, Gregorymys (“Gregory’s mouse”). It must be pretty successful as a burrowing animal, because we find it all over the western United States in the Early Miocene.
Palaeocastor
A hundred meters (300 feet) more and we’ll show you the surprise of the day. Here we are in what looks like a prairie dog town. But those aren’t prairie dogs. They’re a little larger, and quite unfamiliar by modern standards. Can’t guess what they are? These are beavers—Palaeocastor (“ancient beaver”) to be exact. Here in the Early Miocene of North America, beavers don’t build dams. In fact they live neither at the water’s edge nor, like muskrats, in the water. They dig deep, spiral burrows in well drained ground. Some of their burrows are 2.5 meters (8 feet) deep, but 2 meters (6.5 feet) is about average. Down and around and around the burrows go, like giant corkscrews, always ending in straight shafts slanting slightly upward so that living chambers will not be flooded by rainwater running down the burrows.
Paleontologists have called the preserved burrows “devil’s corkscrews”—Daemonelix—since the time they were first found. At first, scientists thought they might be holes left by the giant tap roots of some unknown plant. But when Palaeocastor skeletons were found in the bottoms of the spirals, almost everyone had to concede that they were truly beaver burrows. Admittedly, the skeleton of a Nothocyon was found in one burrow; but this predator probably followed a beaver home for supper and just stayed. Three other kinds of beavers lived around Agate in the Early Miocene, but their bones have never been found in the burrows. No one knows what they did for homes: perhaps their burrows were much shallower or were in the river banks where running water soon destroyed them.
Near the river bank in some soft sand is a nest of tortoise eggs. The hot sun has brought the babies out of their shells and they’re stumbling off in all directions. Right now the biggest is only about twice the size of a silver dollar; but when they’re grown they’ll be about 60 centimeters (24 inches) across the shell, or perhaps even larger. They’re strict vegetarians, grazing and browsing on soft plants and leaves. There are probably some pond turtles around too, but we’ve never seen any.